Hmmm....see post #11 above:
"Ceramic hybrid plates: Most ceramic hybrid plates combine ceramic materials like silicone carbide, aluminum oxide, or boron carbide along with various other materials, including metal, compressed polyethelene, etc... These plates are often heavier than pure ceramic plates, but are a bit more robust, with potentially better multi-hit protection and greater durability. Nonetheless, periodic non-destructive analysis is recommended because of the ceramic elements. Ceramic hybrid designs are often used in stand-alone level IV plates intended to stop AP projectiles."